4 Answers
4

Though I haven't played in a while, here are some pointers I can think of:

Remember how much hits each zombie can survive. This takes some trial and error, but once you get the feel you can make wiser decisions as to your zombie placements. An imp, for example, can walk over thorns and live (two hits, if I remember correctly), but the next hit will make him drop dead (again).

Maximize your use of the dancer zombie. This guy can save you a lot of sun if used correctly. Optimally, you can clear three lanes with minimal casualties by placing him in any non-extreme lane (not topmost nor bottommost). Of course, you would have to make sure to eliminate all ranged opposition in the lane you actually place him on and be wary of plants with split fire (such as the star fruit). You don't have to worry about squashes or chompers since they will waste themselves on the summoned zombie dancing directly in front of the dancer. I also remember that when first placed and performing his moonwalk, this zombie can simply bypass plants. Learn to use this to your advantage.

Sometimes placing two cheaper
zombies can have the same effect as
placing one stronger zombie (which
costs more than both the cheaper
ones). For instance, if you have a
snow pea shooting at you from deep
in a lane, two plain zombies might
be able to reach it, alternating the
shot damage between them as one
thaws and the other doesn't.

Scaredy shrooms cower in fear even when a zombie approaches them on an adjacent lane (not only on their actual lane). Timing your zombie placement right, you can cash in on this behaviour.